FACIES ANALYSIS OF A CARBONATE MUD-MOUND IN THE UPPER SILURIAN WABASH FORMATION, NORTH-CENTRAL INDIANA

The Upper Silurian Wabash Formation in north-central Indiana contains a classic outcrop of a carbonate mud-mound that is exposed along a railroad cut near the historic Big Four Railroad Station in Wabash, Indiana. Despite over 100 years of investigation, many issues surrounding the origin of the mud-mound remain a mystery. This project involved a microfacies analysis of the mound core, proximal flanks, and distal flanks to identify the physical and biological processes responsible for mound development.

The Wabash mound is over 100m wide, and 15m of section are exposed. It contains a highly dolomitized core facies composed of mudstone and skeletal wackestone with a reticulated stromatactis cavity network. The individual cavities range up to 10cm in height, are over 1m long in outcrop, and makes up 45-50% of the core facies. Potential frame-building metazoans are rare in the mound. The abundance of mud playing a constructional role in the mound suggests a microbial origin, but direct evidence of microbes is rare. Sponges are present, but are typically associated with ostracods and laminated mudstones, and are interpreted to occur within large syndepositional cavities within the mound. The flank beds are symmetric about the core and record depositional dips up to 40°. The proximal flank beds are composed of a thick-bedded skeletal packstone. They grade laterally into distal flanks which are composed of thinner-bedded skeletal wackestone. Proximal and distal flanks contain a variety of organisms including, crinoids, bryozoans, ostracods, gastropods, brachiopods and sponge spicules. Photozoans are absent. Stromatactis is less common in the flanks, and is of smaller dimensions than in the core.

These observations suggest microbes were responsible for generating the mud that composes the mound. The absence of photozoans and the lack of current-related directional off-mound sediment transport suggest a deep-water origin for the portion of the mound exposed in the outcrop. Stromatactis is interpreted to represent a cavity network constructed during the early growth of the mounds by microbes and possibly sponges. The cavities contain their own cryptic biota and have been modified by internal erosion, deposition of internal sediment, and marine cementation.